How do I rig this so the wings are symmetrical?
In Edit mode, copy your "wing_L" bone with the Symmetrize function to automatically create a "wing_R" bone.
Switch to Pose mode, select the right wing and give it a Copy Rotation constraint.
Copy the exact same setup as the one on the screenshot: Enable X, Y and Z axis, enable the Invert option of Y and Z, choose Space > Local Space / Local Space.
It should work.




You could also simply, in Pose mode, copy the left bone pose with ctrl and paste to the right bone with ctrlshiftV. Just don't forget to insert a keyframe for the right bone (i).